Networks need timing signals to operate, namely a common clock to reference when transmitting sampled data. For example, voice and video transmission and recovery require a common clock. Traditionally, the common clock in telecom networks is a stratum level clock, such as a building integrated timing source (BITS) or the like. Clock distribution, such as with a stratum clock, is the foundation of all telecom networks with stratum levels referring to the quality of the clock. The ANSI Synchronization Interface Standard T1.101 defines profiles for clock accuracy at each stratum level, as does ITU standard G.810, and Telecordia/Bellcore standards GR-253 and GR-1244. These specifications ensure reliability and levels of accuracy of clocks in telecom networks. For example, a stratum 1 clock can be distributed to nodes equipped with a stratum 2 clock which in turn distribute the clock to nodes with a stratum 3 clock, etc.
Referring to FIG. 1, current timing distribution in an enterprise private network or service provider network 10 flows from a clock source 13 to multiple interconnected nodes, such as switch elements 11 and edge elements 12. Switch elements 11 and edge elements 12 each can include routers, switches, multi-service provisioning platforms (MSPPs), SONET/SDH network elements, and the like. A service requestor 14 can also include the same type of elements as the switch and edge elements 11,12. The clock source 13, such as a stratum clock or the like, can be located outside or within the network 10. The clock source 13 is configured to provide a clock reference signal to elements 11,12,14, which in turn carry the clock reference signal as part of the transmission link, such as within the SONET/SDH frame. In the exemplary network 10 of FIG. 1, the clock source 13 distributes the clock reference signal to the various elements 11,12,14. Further, the network 10 can distribute the clock reference signal outside the network 10 to other elements 14 through the edge elements 12.
Ethernet is becoming the common transport technology of choice in telecom networks; however it is unable to carry a true clock, such as a stratum clock, since Ethernet is asynchronous. Service providers would like to evolve their network to a unified Ethernet network, without T1/E1's, SONET/SDH, etc. This evolution includes replacing T1/E1 backhauling and cross-connecting, such as in the example application of digital loop carrier (DLC) backhaul over Ethernet, from the current DLC backhaul over T1, or such as cell tower backhaul, or such as enterprise access. Disadvantageously, removing SONET/SDH and T1s/E1s from the network eliminates the current mode of timing distribution using stratum clocks with T1/E1 reference signals.
Currently, Ethernet can support soft clock distribution through an adaptive clock over pseudowire emulation edge to edge (PWE3). However, not all service providers trust this adaptive clock (especially over Ethernet) since these clocks do not have the quality or reliability of direct clocks. For example, one conventional means of adaptive clock recovery employs adapting a local clock that is based on the level of the receiver's jitter buffer. Disadvantageously, this method requires a long period before it can lock onto a source clock and is susceptible to buffer overflow and underflow conditions.
Additionally, Optical Transport Network (OTN) multiplexing is an alternative to SONET/SDH multiplexing, and its simplified justification scheme provides a number of advantages when multiplexing 2.5 Gbps signals and above. OTN multiplexing does not, however, address lower-rate signals (such as Gigabit Ethernet (GbE), T1, T3, etc.), which would require multiplexing into SONET/SDH or other frame formats before being handled by an OTN multiplexer. The simplified OTN frame format, multiplexing scheme, and justification methodology provide for a more straightforward and scalable hardware design than SONET/SDH methodologies.
For service providers wishing to provide wavelength services or Ethernet services, OTN multiplexing provides a simpler, more straightforward, and ultimately more cost-effective, transport mechanism than SONET/SDH. However, OTN multiplexing is limited by the ITU-T specifically excluding network synchronization between network elements (NEs) from OTN specifications.
Specifically, OTN does not provide for synchronization of network elements. In ITU-T Recommendation G.8251, “The Control of Jitter and Wander within the Optical Transport Network (OTN)”, it states that the OTN physical layer is not required to transport network synchronization. More precisely, neither the ODUk nor any layers below it are required to transport synchronization. Further, it states that OTUk interfaces are not synchronization interfaces. G.8251 states that the transportation synchronization over SDH is adequate for OTN. In the ITU OTN Tutorial (available at www.itu.int/ITU-T/studygroups/com15/otn/OTNtutorial.pdf), Section 13 states that an OTN NE does not require synchronization interfaces, complex clocks with holdover mode or SSM processing.
OTN does not have mechanisms in place to carry timing without carrying embedded SONET/SDH signals which have an embedded clock. Since Ethernet and OTN are asynchronous media and not configured to carrying synchronization, common clocks are lost. This presents challenges for service providers in evolving their network to a unified Ethernet network without T1's, SONET, etc.